Optical arrangements and apparatus

ABSTRACT

Optical arrangements are described in which an optical beam is reflected by means of a mirror on to a working surface, and is caused to be deflected over the working surface but is automatically maintained continuously in focus on the working surface during such deflection. In some embodiments, the mirror is a plane mirror which is movable about two transverse axes to deflect the beam over the surface, and the optical beam is directed on to the plane mirror via a focussing element, such as a convex lens or mirror, which is linearly movable to maintain the focus on the working surface. In other embodiments, the mirror is fixed and concave, and the optical beam passes through an axial aperture in the mirror and strikes a convex mirror which reflects it on to the concave mirror and thence on to the working surface. The convex mirror is angularly movable about two transverse axes to deflect the beam over the working surface and is also linearly movable along the axis of the concave mirror to maintain the focus.

j United State: 1111 3,797,908 Ward et al. Mar. 19, 1974 1 OPTICALARRANGEMENTS AND 3,411,852 11/1968 Marinozzi 350/294 APPARATUS 2.964.59012/1960 Gillette 350/187 3.533.685 10/1970 Littmann et a1. .1 350/202[75] Inventors: Brooke Armitage Ward,

Goring-on-Thames; David A. Reynolds, Didcot, both of England [73]Assignee: United Kingdom Atomic Energy Authority, London, England [22]Filed: Feb. 22, 1973 [21] Appl. No.: 334,711

Related US. Application Data [63] Continuation of Ser. No. 121,921,March 8. 1971,

abandoned.

[30] Foreign Application Priority Data Mar. 11, 1970 Great Britain[1774/70 [52] US. Cl 350/7, 350/285, 350/294, 178/7.6 [51] Int. Cl. G02b17/00 [58] Field of Search 350/6, 7, 55. 41, 187, 350/202, 294, 285;356/24; 178/76; 95/15 56] References Cited UNITED STATES PATENTS1.136.761 4/1915 Becker 95/15 3.447.859. 6/1969 Barlow 350/7 3.532.42510/1970 Silverberg.... 350/6 2.454.959 11/1948 Barnes 356/24 PrimaryExaminer-David Schonberg Assistant E.raminerMichael J. Tokar Attorney,Agent, or Firm-Larson, Taylor & Hinds [5 7] ABSTRACT Opticalarrangements are described in which an optical beam is reflected bymeans of a mirror on to a working surface, and is caused to be deflectedover the working surface but is automatically maintained continuously ii focus on the working surface during such deflection. In someembodiments, the mirror is a plane mirror which is movable about twotransverse axes to deflect the beam over the surface, and the opticalbeam is directed on to the plane mirror via a focussing element, such asa convex lens or mirror, which is linearly movable to maintain the focuson the working surface. In other embodiments, the mirror is fixed andconcave, and the optical beam passes through an axial aperture in themirror and strikes a convex mirror which reflects it on to the concavemirror and thence on to the working surface. The convex mirror isangularly movable about two transverse axes to deflect the beam over theworking surface and is also linearly movable along the axis of theconcave mirror to maintain the focus.

22 Claims, 14 Drawing Figures PATENTED HAR 1 9 I974 SHEET 1 OF 9PATENTEU HAR 1 9 1974 SHEET 3 OF 9 PATENIEDHAR 1 9 1914 I 3; 7971908SHEET 5 OF 9 PATENTED HAR l 9 I974 SHEET 9 BF 9 OPTICAL ARRANGEMENTS ANDAPPARATUS This is a continuation, division, of application Ser. No.121,921 filed Mar. 8. 1971 now abandoned.

BACKGROUND OF THE INVENTION This invention relates to optical apparatus.Optical apparatus embodying the invention may advantageously be used forscanning a laser beam over a working surface but is by no means limitedto such an application.

Optical apparatus for scanning a working surface is known in which aplane mirror is arranged to be tiltable to and fro to carry out thescan. In such an arrangement. however, the optical path length from theplane mirror to the working surface may vary during the scan, and aproblem of continuously maintaining the scan in focus arises.

In an attempt to overcome this problem, an optical arrangement has beenproposed in which the plane mirror is moved bodily to and fro as ittilts, so as to compensate for the change in path length caused by thetilting.

Such an arrangement may, however, involve complexities owing forexample, to the difficulty of controlling the tilting and bodilymovement of the plane mirror, which may be of relatively large size, andthese complexities may reduce the possible scanning speed for a givenscanning area.

An object of the invention, therefore, is to provide improved opticalapparatus.

A more specific object of the invention is to provide improved opticalapparatus which scans a working surface and maintains the scancontinuously in focus without the necessity of bodily movement ofa planemirror.

Another object of the invention is to provide improved optical apparatuswhich has reduced or zero spherical aberration.

A further object of the invention is to provide improved opticalapparatus which scans a working surface and maintains the scancontinuously in focus and does not use a plane mirror.

BRIEF SUMMARY OF THE INVENTION According to the invention. therefore,there is provided an optical arrangement, comprising optical meansoperative to scan a working surface and including a movably mountedvergencecausing optical focussing element for focussing the scan andcontrol means operative to adjust the focussing element in dependence onthe instantaneous deflection of the point of scan from a predetermineddatum and on the contour of the working surface whereby to maintain thescan continuously in focus on the working surface.

According to the invention, there is further provided an opticalarrangement, comprising optical means operative to scan a workingsurface of a predetermined contour, at least part of the optical meansbeing movable about two transverse axes whereby to deflect the point ofscan to different points on the said surface and at least part of theoptical means being movable so as to adjust the focussing of the scan onthe working surface, and control means operative to control thedifferent movements of the optical means whereby to maintain the scancontinuously in focus on the working surface.

Optical apparatus embodying the invention can be constructed to carryout a scan of a working area of 6 feet across, say, at a speed in therange 100 to 500 feet per minute.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Optical arrangements andapparatus embodying the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. I is a diagrammatic view of one of the optical arrangements;

FIG. 2 is a perspective view of optical apparatus embodying thearrangement of FIG. 1;

FIG. 3 is a plan view of the apparatus of FIG. 2;

FIG. 4 is a side elevation of the apparatus of FIG. 2;

FIG. 5 is a section on the line A-A of FIG. 4; and

FIGS. 6 to 14 show modifications of the arrangement of FIG. 1.

The optical arrangements and apparatus to be described are for carryingout a focussed scan of a working surface. The optical arrangements andapparatus to be described are particularly useful for focussing andscanning a laser beam, such as an infra-red beam from a C0, laser forexample, over the surface of a workpiece in order to carry out amaterialprocessing operation such as cutting, welding, scribing, and thelike, though they may be used to handle other types of optical beams andmay be used where the scanning process picks up a beam from the surfaceinstead of vice versa. The surface is, in the particular examples beingdescribed, a flat or two-dimensional surface, but need not necessarilybe so. Optical apparatus in at least one of the forms to be describedcan be constructed to carry out scans of a working surface 6 feet acrossat a speed in the range 100 to 500 feet per minute.

As shown in FIG. 1, the input beam 15, such as from the laser forexample, is directed on to a fixed spherical mirror 16 by an opticalfocussing element in the form of a corrected lens 18 which is movable toand fro in the directions of the arrows A. The beam 153 reflected fromthe mirror 16 strikes a plane mirror 20 and is deflected thereby so thatthe exit beam 15C strikes the working surface 22. The mirror 20 issupported (by means'not shown) ss as to be angularly movable about afixed axis 24 and to be angularly movable in the directions of thearrows B about a second axis which is perpendicular to the axis 24 andlies in the plane of mirror 20. These angular movements of the mirror 20cause the beam to scan over the surface 22. When the angle dz is thebeam 15C strikes the surface 22 at the origin of an (r,0) coordinatesystem, that is, r 0 when d: 90. Variation of the angle 42 varies theradial coordinate r, while angular movement of the mirror 20 about theaxis 24 varies the angular coordinate 0.

In accordance with a feature of the invention, the lens 18 is maintainedcontinuously axiallyv positioned so that the beam 15C is brought to afocus on the surface 22 irrespective of the angular position andattitude of the mirror 20. In the arrangement of FIG. 1, where thesurface 22 is perpendicular to the chief ray of the beam 15C when thebeam strikes the surface at the origin of the required scanning area,the required position of the lens 18, in order to achieve continuousfocussing, is a function of the angle 4 only. Thus, simple mechanicallinkage, such as involving a cam and cam follower for example, can beprovided between the mirror 20 and the lens 18 so as to move the lens inthe direction of one or other of the arrows A as the angle 4) changes.Such an arrangement is shown more specifically in FIGS. 2 to 5 to bedescribed hereinafter. Instead of such mechanical linkage, however, anelectrical or electromechanical arrangement can be used. For example, atransducer may be mounted to sense the angular movement of the mirror 20about axis 24, and a second transducer may be mounted to sense the valueof the angle B. These two transducers feed a control unit which producesa control signal in dependence thereon. This control signal feeds adrive unit which positions the lens 18 accordingly. The variation of thecontrol signal with the inputs from the transducers is such that themovement of the lens 18 maintains the beam continuously in focus on thesurface.

The path traced out by the focus of the beam C as the mirror 20 isrotated about axis 24 (the angle d: being maintained constant) is anellipse the eccentricity of which is a function of the angle X betweenthe chief rays of the beams 15B and 15C. This can be corrected (that is,the focus can be made to describe a circular path as the mirror 20 isrotated about axis 24) by arranging the mirror-controlling mechanism sothat the angle do is continually adjusted as a function of the angularposition of the mirror 20 about the axis 24.

Apparatus embodying the optical arrangement of FIG. 1 will now bedescribed in detail with reference to FIGS. 2 to 5 in which partscorresponding to parts in FIG. 1 are similarly referenced.

The apparatus of FIGS. 2 to 5 comprises a base 24 supporting anupstanding framework 26 having two legs 28 and 30 and a top 32. The legs28 and 30 are respectively integral with side members 34 and 36 whichare attached to an inclined apertured plate 38 fixed to the front ofthebase 24. The leg 28 and its side member 34 are not shown in FIG. 4. Aplatform 40 is rigidly supported between the legs 28 and 30 and has anupstanding apertured end plate 42. A hollow tubular structure 44 ismounted between the legs 28 and 30 by means of adjustable bolts 46 and48, which engage blocks fast with the platform 40, and by means of afifth bolt 50 which is threadably attached to the top 32. The ends ofthe bolts 46 to 50 support freely rotatable balls thus enabling thestructure 44 to be axially movable. At one end. the structure 44supports the lens 18 (see FIG. 1), and at its other end the structure 44is attached to the end plate 42 by tension springs 52.

The inclined plate 38 supports the plane mirror 20. A shaft 54.rotatablysupported in the plate 38, carries a fork arrangement 56 whichrotates with the shaft and carries the mirror 20. The inner end of theshaft 54 is rigid with a pinion 58 which engages a worm gear 60, thelatter being rotatable by means of a knurled knob 62. The mirror 20 issupported by the fork arrangement 56 so as to be tiltable about an axisthrough thetwo ends of the fork arrangement and thus to vary the angle(1) (see FIG. 1). In order to control the movement of the mirror aboutthis axis, the rear of the mirror is attached by a pivotal link 64 to acollar 66 which is slidable to and fro along the axis of the shaft 54but rotates with the shaft. A spring-loaded plunger 68 engages-the rearof the mirror 20 and urges the collar 66 upwards. A bracket 70 supportsa rotatable wheel 72 carrying an eccentric pin 74 (FIG. 3). The wheel 72is turned by means of pivotally linked levers 76 and 78 which areattached to a bell crank 80. The bell crank 80 is angularly rotated bymeans of a pinion 82 and a worm gear 89, the latter being carried on ashaft 86 which protrudes through a hole in the base 80 and is rigid witha knurled knob 88. Rotation of the knob 88 causes corresponding rotationof the wheel 72. The eccentric pin 74 bears on the collar 66 and, incombination with the spring-loaded plunger 68, adjusts the axialposition of the collar and thus the angle d: of tilt of the mirror 20.

The bell crank 86 is formed with a cam surface 90 which engages a camfollower on the end of a pivoted lever 92. The upper end of the lever 92is forked and pivotally attached to the structure 44.

At its end opposite to the mirror 20, the base 80 supports an inclinedplane mirror 94.

The spherical mirror 16 of FIG. 1 is not attached to the base 80 but ismounted separately therefrom (not shown in FIGS. 2 to 5).

In operation, a laser beam from a source not shown is directed onto themirror 94 so as to be reflected through the tubular structure 44 and thelens 18. The beam 15A emerging from the lens 18 is reflected by thespherical mirror 16 on to the mirror 20 and thence on to the workingsurface in the manner explained in connection with FIG. 1. The 0coordinate of the position of the focussed beam 15C on the workingsurface is controlled by rotation of the knurled knob 62. while the rcoordinate is controlled by rotation of the knurled knob 88. As the knob88 is turned to adjust the tilt of the mirror 20, the engagement of thelever 92 with the cam surface 90 causes the structure 44 to be movedlongitudinally so as to locate the lens 18 in the desired position tomaintain the beam 15C continually in focus on the working surfacesTheprofile of the cam surface 90 is designed to give the desiredrelationship between the tilt of the mirror 20 and the axial position ofthe lens 18 having regard to the configuration of the work ing surface(a sine relationship is used for a flat working surface).

The worm gears 60 and 84 can be split and sprungloaded to reducebacklash.

The arrangement and apparatus described with reference to FIGS. 1 to 5can be modified so as to focus the beam 15C on to a non-planar workingsurface. This modification is achieved by altering the interrelationshipbetween the angle of the tilt d1 of the mirror 20 and the axial positionof the mirror 18. Thus, in the apparatus described with reference toFIGS. 2 to 5, this can be achieved by altering the profile of the camsurface 90. If the surface 22 is not perpendicular to the chief ray ofthe beam 15C when the beam strikes the surface at the origin of thescanned area, or if the contour of the surface so requires, the positionof the lens 18 must be continuously adjusted as a function of both theangle d and the angular position of the mirror about the axis 24involving somewhat more complicated interconnecting linkage between themirror-controlling mechanism and the lens 18. In other apparatus. theangle of tilt 41, the angle of the mirror 20 about axis 24, and theaxial position of the lens 18 can be separately controllable bynumerical control means or other programmable control means which arearranged to maintain the lens 18 in the desired axial position at anytime having regard to the shape of the working surface. Instead,automatic control means may be provided in the form, for example, of acontactless surface sensor on the working surface 22 which senses theincident beam may become important in larger aperture systems (when theaperture exceeds about f/lO).

FIG. 6 illustrates a modification of the arrangement of FIG. 1 designedto reduce the aberrations in the FIG. 1 arrangement. In FIG. 6, thespherical mirror 16 is replaced by an elliptic mirror 100 which ispositioned off axis (its axis 102 being shown dotted) to receive thebeam A. This modification reduces the aberrations encountered with thespherical mirror 16 and simplifies construction somewhat.

FIG. 7 shows a further modification which eliminates the aberrationcaused by the mirror 16 of FIG. 1. In FIG. 7, the spherical mirror 16 isreplaced by an on-axis spherical mirror 104. The movable lens 18 isreplaced by a movable concave mirror 106 which receives the input beam15 through an axial aperture in the mirror 104. The mirror 106 is, likethe lens 18 of FIG. 1, movable to and fro in the directions of thearrows A and is linked with the mirror so as to be moved to and fro incorrespondence with the instantaneous position of the mirror 20 in themanner explained in connection with FIG. 1. The modification of FIG. 7causes some obscuration of the beam 15C.

In the modification shown in FIG. 8, the on-axis spherical mirror 104 ofFIG. 7 is replaced by an on-axis elliptical mirror 108. The lens 18 ofFIG. 1 is, in the modification of FIG. 8, replaced by a small parabolicmirror 110 which is movable to and fro, in correspondence with thesetting of the mirror 20, in the manner explained with reference tomirror 106 of FIG. 7.

In the modification shown in FIG. 9, the spherical mirror 16 of FIG. 1is replaced with an on-axis aspherical mirror 114. In addition, however,both the lens 18 and the plane mirror 20 of FIG. 1 are replaced by a Themodifications of FIGS. 9 and 10, by combining the deflecting action ofthe mirror 20 of FIG. 1 with the focussing action of the lens 18,eliminate the need for a large deflecting mirror with its consequentlimitations on high scanning speeds.

FIGS. 11 to 14 show different optical constructions which can be usedinstead of the final deflecting mirror 20 of FIG. 1.

In FIG. 11, the mirror 20 is replaced by a plane mirror 124 which isused instead of the mirror 20 to reflect the beam 15C on to the workingsurface 22. The mirror small aspherical convex mirror 116. The mirror116 receives the incident beam 15 and reflects it onto the mirror I14whence it is deflected to the surface 22. The mirror 116 is pivotableabout an axis through the point 118 so as to move in the direction ofthe arrow C: this movement defines the r coordinate of the position ofthe focussed beam 15C on the surface 22 and corresponds to the tiltingof the mirror 20 in the FIG. 1 arrangement. In addition, the mirror 116is angularly movable about the axis 120 which corresponds to the axis 24of FIG. 1 and defines the 0 coordinate ofthe position of the beam 15C onthe surface 22. Finally, the mirror 116 is movable to and fro along theaxis 120: this movement corresponds to the axial movement of the lens 18of FIG. 1 and, in similar fashion to that explained in connection withFIG. 1, is linked with themgular movement of the mirror so as tomaintain the beam continually in focus on the surface 22 irrespective ofits position thereon.

FIG. 10 shows a modification of FIG. 9 in which a corrector plate 122 isinterposed between the mirror 116 and the mirror 114. The refractioncaused by the corrector plate 48 enables the system to be designed withreduced parallax error; that is. the beam 15C has a near-perpendicularincidence on the surface 22.

124 is pivotally mounted by bearings 126 inside a frame 128 which isitself pivotally mounted, by bearings 130, in a base framework (notshown). The mirror 124 is thus tiltable about two perpendicular axes bymeans of a control arm 132, and this tilting movement deflects the beam15C along Cartesian coordinates on the working surface 22.

In FIG. 12, the mirror 20 of FIG. 1 is replaced by two mechanicallylinked plane mirrors 134 and 136. These mirrors are rotatable about theaxis 24 so as to define the 0 coordinate of the position of the beam 15Con the working surface, and are tiltable (together) about aperpendicular axis in the directions of the arrows B so as to define ther coordinate.

In FIG. 13, two independently movable plane mirrors 138 and 140 are usedinstead of the mirror 20. Mirror 138 is tiltable about an axis 142,while mirror 140 is tiltable about the perpendicular axis 144. Themirrors 138 and 140 thus control the beam position on the surface 22according to Cartesian coordinates, the position of mirror 138 relativeto its axis defining the Jr coordinate and the position of the mirror140 relative to its axis defining the y coordinate.

In FIG. 14, a deflectable prism 146 replaces the mirror 20. The prismreflects the incident beam 158 by total internal reflection. The prismis supported for rotation about the axis 24 so as to define the Ocoordinate of the position of the beam 15C on the surface 22, and istiltable about a perpendicular axis in the directions of the arrows B soas to define the r coordinate.

In each of the arrangements illustrated in FIGS. 6 to 14, the controlmeans for automatically maintaining the beam in focus on the workingsurface 22 as it scans across the surface can be the same as thatdescribed with reference to FIGS. 1 to 5. Thus, for example, it cancomprise mechanical linkage of similar type to that shown in FIGS. 2 to5, or it can comprise an electromechanical system as described withreference to FIG. 1 in the latter case there would be a respectivetransducer for sensing the movement of the beam-deflecting element ineach of its two axes, the transducers being arranged to feed a controlunit which produces a con-- trol signal for correctly positioning thefocusdetermining element (in the arrangements of FIGS. 9 and 10, thebeam-deflecting and focus-determining elements are the same). Instead.as described the control means could comprise a contactlessphotosensitive sensor on the working surface 22 which senses theincident beam and automatically positions the focusdetermining elementso as to maintain the incident beam continuously in focus on thesurface.

The control means may alternatively take the form of a firstprogrammable control unit for positioning the beam-deflecting element ofthe arrangement according to a predetermined program, and a secondprogrammable control unit for positioning the focus-determining elementaccording to a predetermined program, the two programs beingpredetermined such that the beam is automatically maintained in focus onthe working surface continuously.

It will be appreciated that the term in focus" as used herein is to beinterpreted as meaning at least substantially in focus.

In the arrangements of FIGS. 6, 7, 8, 9 and 10, the focussing element(the lens 18 or the mirrors 106, 110 and 116) can be mounted in a tubeof the type shown in the structure 44 in FIGS. 2 to 5, this tube beingmounted for linear movement to provide the focussing action. The mirror116 in FIGS. 9 and is required to be movable about two transverse axesas well so as to deflect the beam over the working surface, and thesemovements could be accommodated by pivotally mounting the mirror 116 inthe tube and also allowing the tube to turn about its longitudinal axis.The mirrors 20 of FIGS. 6, 7 and 8 can be moutned in similar fashion tothat shown in FIGS. 2 to 5.

What is claimed is: 1. Optical apparatus comprising a base, meansdefining a working surface of predetermined contour fixedly positionedrelative to the base,

means defining a first optical beam path whose direction is fixed inthree dimensions relative to the base,

optical means mounted on the base between the said first path and theworking surface to receive an optical beam which also passes along thesaid first path, the optical means being operative to scan the beamacross the working surface and including a movably mountedvergence-causing optical focussing element for focussing the scan, and

control means operatively connected to the focussing element to adjustthe focussing of the scan on the working surface in dependence on itsdeflection from a predetermined datum whereby to maintain the scancontinually in focus on the working surface.

2. Apparatus according to claim 1, in which the optical means comprisesplanar mirror means in addition to the optical focussing element andmounted to define a second optical path between the focussing elementand the surface,

first mounting means mounting the optical focussing element for linearmovement along one said path to focus the scan, and

second mounting means mounting the mirror means for continuous angularmovement about at least a first axis to deflect the point of scan acrossthe said surface.

3. Apparatus according to claim 2, in which the second mounting meansincludes means mounting the mirror means for angular movement about asecond axis transverse to the first axis.

4. Apparatus according to claim 3, in which the planar mirror meanscomprises a single plane mirror, the second mounting means comprisingmeans mounting the mirror for angular movement about an axissubstantially in the plane of the mirror and constituting the said firsttransverse axis,

and means mounting the mirror for angular movement about an axisperpendicular to the first axis and transversely intersecting the planeof the mirror and constituting the said second transverse axis,

whereby the angular position of the mirror about the first axisdetermines the r coordinate, in a (r,) coordinate system, of theposition of the point of scan on the working surface and the angularposition of the mirror about the second axis determines the 0coordinate. 5. Apparatus according to claim 3, in which the planarmirror means comprises a single plane mirror, and in which the secondmounting means comprises means mounting the mirror for angular movementabout two axes lying substantially in the plane of the single mirror andperpendicular to each other and constituting the said first and secondtransverse axes, whereby the angular position of the mirror about one ofthe axes establishes the x coordinate, in a Cartesian coordinate system,of the position of the point of scan on the working surface and theangular position of the mirror about the other of the two transverseaxes establishes the y coordinate.

6. Apparatus according to claim 3, in which the planar mirror meanscomprises two rigidly interconnected plane mirrors mounted parallel toeach other with their reflecting surfaces facing whereby the said secondoptical path passes by reflection from one mirror to the other, the saidfirst transverse axis being defined between the two mirrors and parallelto the planes thereof whereby the angular position of the mirrors aboutthe first axis establishes the r coordinate, in an (r,0) coordinatesystem, of the position of the point of scan on the working surface, andthe said second transverse axis being perpendicular to the first axiswhereby the angular position of the mirrors about the second axisestablishes the 0 coordinate.

7. Apparatus according to claim 3, in which the planar mirror meanscomprises first and second plane mirrors mounted with their reflectingsurfaces facing whereby the said second optical path passes byreflection from one mirror to the otherand in which the said secondmounting means comprises means mounting the first mirror for movementabout an axis which is substantially in the plane of the first mirrorand constitutes the said first transverse axis and about which the firstmirror is angularly movable whereby to establish the x coordinate, in aCartesian coordinate system, of the position of the point of scan on theworking surface, and

means mounting the second plane mirror for independent angular movementabout an axis which is perpendicular to the first axis and liessubstantially in the plane of the second mirror and about which thesecond mirror is angularly movable to establish the y coordinate of theposition of the point of scan on the working surface.

8. Apparatus according to claim 3, in which the planar mirror meanscomprises a prism mounted whereby the said optical path passestherethrough by total internal reflection, the first mounting meanscomprising means defining the said first transverse axis as an axispassing through the prism and about which the prism is angularly movablewhereby to establish the r coordinate, in a (r,0) coordinate system, ofthe point of scan on the working surface, and

means defining the said second transverse axis as an axis through theprism and perpendicular to the first axis and'about which the prism isalso angucal means also includes a fixed mirror, and

means mounting the fixed mirror in the said second optical path andbetween the focussing element and the planar mirror means.

'10. Apparatus according to claim 9, in which:

the said focussing element is a convex lens, and

the fixed mirror is a concave mirrormounted off-axis with respect to thesaid second optical path from the convex lens.

11. Apparatus according to claim 9, in which:

the said fixed mirror is a concave mirror defining an axial aperturetherethrough, and

the focussing element is a convex mirror mounted to face the axialaperture and whereby the said linear movement thereoftakes place alongan axis passing through the said aperture. 12. Optical apparatuscomprising a base, means defining a working surface of predeterminedcontour fixedly positioned relative to the base,

means defining a first optical beam path whose direction is fixed inthree dimensions relative to the base,

a vergence-causing optical focussing element,

means mounting the focussing element on the base between the said firstpath and the working surface to receive an optical beam which alsopasses along the said first path, the said mounting means mounting thefocussing element for movement about first and second transverse axeswhereby to scan the beam across the said working surface, and mountingit also to be movable linearly to adjust the focussing of the scan, and

control means operatively connected to the focussing element to adjustthe focussing of the scan on the working surface in dependence on thedeflection of the scan from a predetermined datum whereby to maintainthe scan continuously in focus on the working surface.

13. Apparatus according to claim 1, in which the optical means comprisesa concave mirror having an axial aperture which defines the said firstoptical beam path, the concave mirror being mounted to define a secondoptical path between the focussing element and the working surface, andin which the focussing element is a convex mirror mounted in alignmentwith the said aperture to face the reflecting surface of the concavemirror to receive and reflect the said optical beam which passes throughthe aperture, the convex mirror being mounted for linear movement to andfro along the axis of the concave mirror to adjust the focus of thescan. and also being angularly movable about the axis of the concavemirror and about an axis perpendicular thereto whereby the angularpositions of the convex mirror about these two axes respectivelyestablish the r and 6 coordinates of the position of the point of scanon the working surface.

14. Apparatus according to claim 1, in which the said control meanscomprises linkage responsive to that movement of the optical means whichdetermines the deflection of the beam on the working surface andconnected to control, in dependence thereon, that movement of thefocussing element which detennines the focussing of the scan on theworking surface.

15. Apparatus according to claim 3, in which the control means comprisesfirst linkage means mounted to sense movement of the mirror means aboutat least one of the said transverse axes,

second linkage means connected to the focussing element mounting meansfor linearly moving the focussing element', and

motion-transmitting means arranged to transmit the movement sensed bythe first linkage means to the second linkage means whereby theresultant linear movement of the focussing element maintains the scan infocus on the working surface.

16. Apparatus according to claim 1, in which the control means comprisesfirst programmable control means connected to control thedeflection-determining movement of the optical means according to apredetermined program, and second programmable control means connectedto control the focussing-determining movement ofthe focussing element toa predetermined program,

the two programs being arranged whereby the scan is maintainedcontinuously in focus on the working surface.

17. Apparatus according to claim 1, in which the control means comprisesbeam sensing means on the working surface and operative to produce acontrol signal in response to an incident beam thereon, and

means responsive to the control signal to control thefocussing-determining movement of the focussing element in a manner suchas to maintain the scan continuously in focus on the surface.

18. Apparatus according to claim 1, including a source producing alaser-beam and directing it to the optical means along said firstoptical path for scanning and focussing thereby onto the said surface.

19. Optical apparatus, comprising means defining a working surface,

optical means mounted to perform a focussable scan of the workingsurface, first means mounting at least part of the optical means forrespective angular movements about first and second transverse axeswhereby to deflect the point of scan to different points on apredetermined area of the said surface, which area has a length andbreadth greater than those of the said point of scan,

second means mounting at least part of the optical means for linearmovement so as to adjust the focussing of the scan on the workingsurface, and

control means operative to control the linear, focussing determiningmovement of the optical means in dependence on at least one of theangular deflecting determining movements thereof whereby to maintain thebeam continuously in focus on the working surface.

20. Apparatus according to claim 19, in which the said part of theoptical means mounted by the first mounting means comprises planarmirror means, and the said part mounted by the second mounting meanscomprises an optical focussing element, the planar mirror means definingan optical path between the optical focussing element and the workingsurface.

21. Apparatus according to claim 19, in which the optical meanscomprises an optical focussing element, and in which the first mountingmeans mounts the focussing element for said angular movements about thetwo transverse axes and the second mounting means mounts the focussingelement for said linear movement to adjust the focussing of the scan.

22. Apparatus according to claim 19, including an optical beam sourceand in which:

the optical means comprises a curved mirror defining a central aperturetherethrough and mounted whereby the said optical beam passes throughthe aperture. and a convex mirror of smaller diameter than the concavemirror and mounted to face the concave mirror and in line with theaperture to receive the optical beam therethrough and to reflect it onto the concave mirror and thence on to the said working surface; and

the said first mounting means comprises means mounting the convex mirrorfor angular movement about an axis transverse to the surface of theconcave mirror and passing through the aperture and also for angularmovement about a perpendicular axis; and

the second mounting means comprises means mounting the convex mirror forlinear movement along the said axis transverse to the surface of theconcave mirror and passing through the aperture. I! i I

1. Optical apparatus comprising a base, means defining a working surfaceof predetermined contour fixedly positioned relative to the base, meansdefining a first optical beam path whose direction is fixed in threedimensions relative to the base, optical means mounted on the basebetween the said first path and the working surface to receive anoptical beam which also passes along the said first path, the opticalmeans being operative to scan tHe beam across the working surface andincluding a movably mounted vergence-causing optical focussing elementfor focussing the scan, and control means operatively connected to thefocussing element to adjust the focussing of the scan on the workingsurface in dependence on its deflection from a predetermined datumwhereby to maintain the scan continually in focus on the workingsurface.
 2. Apparatus according to claim 1, in which the optical meanscomprises planar mirror means in addition to the optical focussingelement and mounted to define a second optical path between thefocussing element and the surface, first mounting means mounting theoptical focussing element for linear movement along one said path tofocus the scan, and second mounting means mounting the mirror means forcontinuous angular movement about at least a first axis to deflect thepoint of scan across the said surface.
 3. Apparatus according to claim2, in which the second mounting means includes means mounting the mirrormeans for angular movement about a second axis transverse to the firstaxis.
 4. Apparatus according to claim 3, in which the planar mirrormeans comprises a single plane mirror, the second mounting meanscomprising means mounting the mirror for angular movement about an axissubstantially in the plane of the mirror and constituting the said firsttransverse axis, and means mounting the mirror for angular movementabout an axis perpendicular to the first axis and transverselyintersecting the plane of the mirror and constituting the said secondtransverse axis, whereby the angular position of the mirror about thefirst axis determines the r coordinate, in a (r, phi ) coordinatesystem, of the position of the point of scan on the working surface andthe angular position of the mirror about the second axis determines thetheta coordinate.
 5. Apparatus according to claim 3, in which the planarmirror means comprises a single plane mirror, and in which the secondmounting means comprises means mounting the mirror for angular movementabout two axes lying substantially in the plane of the single mirror andperpendicular to each other and constituting the said first and secondtransverse axes, whereby the angular position of the mirror about one ofthe axes establishes the x coordinate, in a Cartesian coordinate system,of the position of the point of scan on the working surface and theangular position of the mirror about the other of the two transverseaxes establishes the y coordinate.
 6. Apparatus according to claim 3, inwhich the planar mirror means comprises two rigidly interconnected planemirrors mounted parallel to each other with their reflecting surfacesfacing whereby the said second optical path passes by reflection fromone mirror to the other, the said first transverse axis being definedbetween the two mirrors and parallel to the planes thereof whereby theangular position of the mirrors about the first axis establishes the rcoordinate, in an (r, theta ) coordinate system, of the position of thepoint of scan on the working surface, and the said second transverseaxis being perpendicular to the first axis whereby the angular positionof the mirrors about the second axis establishes the theta coordinate.7. Apparatus according to claim 3, in which the planar mirror meanscomprises first and second plane mirrors mounted with their reflectingsurfaces facing whereby the said second optical path passes byreflection from one mirror to the other and in which the said secondmounting means comprises means mounting the first mirror for movementabout an axis which is substantially in the plane of the first mirrorand constitutes the said first transverse axis and about which the firstmirror is angularly movable whereby to establish the x coordinate, in aCartesian coordinate system, of the position of the point of scan on theworking surface, and means mounting the second plane mirror forindependent anguLar movement about an axis which is perpendicular to thefirst axis and lies substantially in the plane of the second mirror andabout which the second mirror is angularly movable to establish the ycoordinate of the position of the point of scan on the working surface.8. Apparatus according to claim 3, in which the planar mirror meanscomprises a prism mounted whereby the said optical path passestherethrough by total internal reflection, the first mounting meanscomprising means defining the said first transverse axis as an axispassing through the prism and about which the prism is angularly movablewhereby to establish the r coordinate, in a (r, theta ) coordinatesystem, of the point of scan on the working surface, and means definingthe said second transverse axis as an axis through the prism andperpendicular to the first axis and about which the prism is alsoangularly movable whereby to establish the theta coordinate. 9.Apparatus according to claim 3, in which the optical means also includesa fixed mirror, and means mounting the fixed mirror in the said secondoptical path and between the focussing element and the planar mirrormeans.
 10. Apparatus according to claim 9, in which: the said focussingelement is a convex lens, and the fixed mirror is a concave mirrormounted off-axis with respect to the said second optical path from theconvex lens.
 11. Apparatus according to claim 9, in which: the saidfixed mirror is a concave mirror defining an axial aperturetherethrough, and the focussing element is a convex mirror mounted toface the axial aperture and whereby the said linear movement thereoftakes place along an axis passing through the said aperture.
 12. Opticalapparatus comprising a base, means defining a working surface ofpredetermined contour fixedly positioned relative to the base, meansdefining a first optical beam path whose direction is fixed in threedimensions relative to the base, a vergence-causing optical focussingelement, means mounting the focussing element on the base between thesaid first path and the working surface to receive an optical beam whichalso passes along the said first path, the said mounting means mountingthe focussing element for movement about first and second transverseaxes whereby to scan the beam across the said working surface, andmounting it also to be movable linearly to adjust the focussing of thescan, and control means operatively connected to the focussing elementto adjust the focussing of the scan on the working surface in dependenceon the deflection of the scan from a predetermined datum whereby tomaintain the scan continuously in focus on the working surface. 13.Apparatus according to claim 1, in which the optical means comprises aconcave mirror having an axial aperture which defines the said firstoptical beam path, the concave mirror being mounted to define a secondoptical path between the focussing element and the working surface, andin which the focussing element is a convex mirror mounted in alignmentwith the said aperture to face the reflecting surface of the concavemirror to receive and reflect the said optical beam which passes throughthe aperture, the convex mirror being mounted for linear movement to andfro along the axis of the concave mirror to adjust the focus of thescan, and also being angularly movable about the axis of the concavemirror and about an axis perpendicular thereto whereby the angularpositions of the convex mirror about these two axes respectivelyestablish the r and theta coordinates of the position of the point ofscan on the working surface.
 14. Apparatus according to claim 1, inwhich the said control means comprises linkage responsive to thatmovement of the optical means which determines the deflection of thebeam on the working surface and connected to control, in dependencethereon, that movement of the focussing element which determines thefocussing of the scan On the working surface.
 15. Apparatus according toclaim 3, in which the control means comprises first linkage meansmounted to sense movement of the mirror means about at least one of thesaid transverse axes, second linkage means connected to the focussingelement mounting means for linearly moving the focussing element, andmotion-transmitting means arranged to transmit the movement sensed bythe first linkage means to the second linkage means whereby theresultant linear movement of the focussing element maintains the scan infocus on the working surface.
 16. Apparatus according to claim 1, inwhich the control means comprises first programmable control meansconnected to control the deflection-determining movement of the opticalmeans according to a predetermined program, and second programmablecontrol means connected to control the focussing-determining movement ofthe focussing element to a predetermined program, the two programs beingarranged whereby the scan is maintained continuously in focus on theworking surface.
 17. Apparatus according to claim 1, in which thecontrol means comprises beam sensing means on the working surface andoperative to produce a control signal in response to an incident beamthereon, and means responsive to the control signal to control thefocussing-determining movement of the focussing element in a manner suchas to maintain the scan continuously in focus on the surface. 18.Apparatus according to claim 1, including a source producing alaser-beam and directing it to the optical means along said firstoptical path for scanning and focussing thereby onto the said surface.19. Optical apparatus, comprising means defining a working surface,optical means mounted to perform a focussable scan of the workingsurface, first means mounting at least part of the optical means forrespective angular movements about first and second transverse axeswhereby to deflect the point of scan to different points on apredetermined area of the said surface, which area has a length andbreadth greater than those of the said point of scan, second meansmounting at least part of the optical means for linear movement so as toadjust the focussing of the scan on the working surface, and controlmeans operative to control the linear, focussing determining movement ofthe optical means in dependence on at least one of the angulardeflecting determining movements thereof whereby to maintain the beamcontinuously in focus on the working surface.
 20. Apparatus according toclaim 19, in which the said part of the optical means mounted by thefirst mounting means comprises planar mirror means, and the said partmounted by the second mounting means comprises an optical focussingelement, the planar mirror means defining an optical path between theoptical focussing element and the working surface.
 21. Apparatusaccording to claim 19, in which the optical means comprises an opticalfocussing element, and in which the first mounting means mounts thefocussing element for said angular movements about the two transverseaxes and the second mounting means mounts the focussing element for saidlinear movement to adjust the focussing of the scan.
 22. Apparatusaccording to claim 19, including an optical beam source and in which:the optical means comprises a curved mirror defining a central aperturetherethrough and mounted whereby the said optical beam passes throughthe aperture, and a convex mirror of smaller diameter than the concavemirror and mounted to face the concave mirror and in line with theaperture to receive the optical beam therethrough and to reflect it onto the concave mirror and thence on to the said working surface; and thesaid first mounting means comprises means mounting the convex mirror forangular movement about an axis transverse to the surface of the concavemirror and passing through the aperture and also for angular movementabout a perpendicular Axis; and the second mounting means comprisesmeans mounting the convex mirror for linear movement along the said axistransverse to the surface of the concave mirror and passing through theaperture.